Heat Loss Through Walls Calculator
Estimate wall heat transfer from dimensions, temperatures, insulation layers, U-values, thermal bridges, and operating time with clear formulas, comparisons, exports, and practical results today.
Calculator Inputs
Metric and imperial supportWall Geometry
Thermal Conditions
Wall Layers
For each layer, choose a preset or enter custom thermal conductivity.
Upgrade Comparison
Formula Used
Layer resistance: R = L / k
Total resistance: Rtotal = Rsi + Σ(L/k) + Rso
U-value: U = 1 / Rtotal
Heat loss rate: Q = U × A × ΔT
Energy loss: E = Q × time
How to Use This Calculator
- Choose layers, a known U-value, or a known R-value.
- Enter wall dimensions and subtract windows or doors.
- Enter indoor and outdoor temperatures.
- For layered walls, enter thickness and conductivity values.
- Add a thermal bridge allowance when appropriate.
- Press Calculate, then review heat rate and energy loss.
- Use CSV or PDF to save the calculation.
Example Calculation
A 12 m² net wall has a U-value of 0.35 W/(m²·K). If indoors is 21°C and outdoors is 5°C, ΔT equals 16 K. The basic heat loss is 0.35 × 12 × 16 = 67.2 W. With a 5% thermal-bridge allowance, the estimate becomes 70.56 W. Over 24 hours, that equals about 1.693 kWh of heat loss.
Typical Wall Data
| Wall type | Typical U-value W/(m²·K) | Approximate R-value m²·K/W | General performance |
|---|---|---|---|
| Solid uninsulated brick | 1.8–2.2 | 0.45–0.56 | High heat loss |
| Uninsulated cavity wall | 1.2–1.6 | 0.63–0.83 | Moderate to high loss |
| Insulated cavity wall | 0.25–0.35 | 2.86–4.00 | Good insulation |
| Insulated timber stud wall | 0.18–0.30 | 3.33–5.56 | Good to very good |
| High-performance insulated wall | 0.10–0.18 | 5.56–10.00 | Very low heat loss |
Values are broad examples. Real assemblies depend on materials, moisture, workmanship, framing, and local standards.
Understanding Heat Loss Through Walls
Heat loss through walls occurs when thermal energy moves toward colder surroundings. The rate depends on wall area, temperature difference, and thermal resistance. A larger temperature difference usually produces faster heat transfer. Insulation slows this transfer by increasing the wall assembly's resistance. This calculator can use layers, U-values, or R-values directly. Layer mode is useful when construction details are available. Each material contributes resistance according to thickness and conductivity. Thick insulation usually adds more resistance than thin insulation. Low conductivity materials also resist heat flow more effectively. Surface resistances represent air films along both wall faces. Thermal bridges create easier paths for heat to escape. Studs, beams, fasteners, and junctions can create these paths. Openings should be deducted when their areas are analyzed separately. Otherwise, the wall area could be overstated significantly. U-value describes heat flow through one square metre. Lower U-values generally indicate better thermal performance. R-value expresses resistance, so higher values indicate stronger insulation. The calculator converts imperial inputs into consistent internal SI units. It then displays familiar output units for practical use. Heat loss rate is power, usually expressed in watts. Energy loss includes time and is commonly measured in kilowatt-hours. This distinction matters when estimating heating costs and seasonal energy use. The upgrade comparison estimates savings from a lower future U-value. It also adjusts input energy using heating-system efficiency. Energy prices can then provide a simple annual cost estimate. Real buildings can behave differently because conditions continually change. Wind, moisture, solar gains, air leakage, and workmanship matter. Thermal imaging can reveal defects that simple calculations cannot capture. Building regulations may also prescribe specific calculation methods and limits. Use this calculator for estimates, comparisons, and preliminary design decisions. Detailed projects should use verified material data and local standards. Better insulation can reduce demand, costs, and discomfort indoors.
FAQ
What is the basic wall heat-loss formula?
Use Q = U × A × ΔT. U is transmittance, A is area, and ΔT is temperature difference.
Should windows and doors be deducted?
Yes, deduct openings when you calculate their heat loss separately. Different components usually have different U-values.
What is the difference between U-value and R-value?
U-value measures heat transmission. R-value measures resistance. For SI units, U equals one divided by R.
What are thermal bridges?
Thermal bridges are conductive paths through insulation. Common examples include framing, slab edges, fasteners, and structural connections.
Can I calculate annual savings after insulation?
Yes. Enter an improved U-value, annual heating hours, efficiency, and energy price to estimate savings.
Are material preset values exact?
No. Presets are representative values. Use manufacturer or laboratory data when accuracy is important.